1. Technical Field
The present invention is directed toward the field of optical communication circuits using laser diodes. More specifically, the invention provides a digital laser driver circuit that is particularly well-suited for use in driving a laser diode for use with an optical communication system. As part of such a system, the laser diode is pulsed on and off at a very high frequency in order to communicate pulses of light over an optical fiber.
2. Description of the Related Art
Laser diodes and their associated driving circuitry are known in this field. Laser diodes are characterized by a temperature dependent threshold current ITH, above which point the diode begins to act like a laser. FIG. 1, for example, is a plot 10 showing the typical light output (P) 12 v. current (I) 14 characteristic for a laser diode at two operating temperatures 16, 18. As seen in this plot 10, the laser diode threshold current ITH is lower ITH1 at the lower temperature than at the higher temperature, where the threshold current is ITH2. The Quantum Efficiency (QE) of the diode is characterized by the slope of the P v. I curve.
Simple laser driver circuits turn the laser diode on and off for each pulse of light to be transmitted over the fiber. FIG. 2 is a plot 20 showing a plurality of light pulses output from such a laser driver circuit. The y-axis in this plot shows light output (P) 22, and the x-axis shows time (t) 24. As seen in this plot 20, the problem with this type of simple on/off driver circuit is that it causes the laser diode to cross over the laser threshold current level (ITH), which causes a ringing phenomenon 26 to occur on the output pulse that consists of a plurality of light spikes. These light spikes are caused by the laser transitioning from operating like a light emitting diode to operating like a laser. After a short burst, these spikes 26 subside, and the output pulse is relatively flat 28, until the pulse terminates.
In order to cure the problem shown in FIG. 2, more complex laser driver circuits have been used in this field that typically include two analog feedback loops. The first analog feedback loop regulates the laser diode""s average light output and maintains the laser above the threshold current level (ITH) even during off periods. This eliminates the ringing phenomenon shown in FIG. 2 since the diode is always above the threshold current. The second analog feedback loop is used to regulate the modulation index, and requires a complex gain control stage to adjust the laser diode""s extinction ratio ER. Often, these analog feedback circuits require temperature compensation thermistors and multiple factory adjustments to control the extinction ratio.
A digital laser driver circuit is provided in which a digital synthesizer synthesizes a modulation index signal for precisely setting the modulation level of the laser, and a pair of feedback loops control the average power level and modulation index of the laser diode. The laser diode includes a back-facet photodiode that is used to monitor the laser""s average and peak optical power levels. The average power level is measured by an analog feedback loop and compared to an externally supplied reference voltage in order to maintain the laser at a particular average optical power level. The peak power is measured by a digital feedback loop and compared to a pair of threshold levels based on a ratio of the average power level using digital comparators. The comparators provide signals to the digital synthesizer in order to indicate that the modulation level is too low or too high. The digital synthesizer then re-synthesizes the laser modulation index signal to maintain the optical modulation between the two threshold levels.
One aspect of the invention provides a digital laser driver circuit for controlling the drive current to a laser diode having a back-facet monitor photodiode, comprising: (1) a current driver coupled to the laser diode; (2) an analog feedback loop coupled between the back-facet photodiode and the current driver for measuring the average power level of light output from the laser diode and for maintaining a constant optical power output level; and (3) a digital feedback loop coupled between the back-facet photodiode and the current driver for comparing the peak power level of light output from the laser diode to a ratio of the average power level and for synthesizing a laser modulation level to the current driver that keeps the peak power level between two average threshold levels.
Another aspect of the invention provides a digital laser communication circuit, comprising: a laser diode; a back-facet photodiode for monitoring the light output from the laser diode; a current driver coupled to the laser diode; a digital synthesizer for creating a modulation index signal, wherein the modulation index signal is coupled to the current driver; an analog feedback loop coupled between the back-facet photodiode and the current driver for maintaining a constant average optical power level from the laser diode; and a digital feedback loop coupled between the back-facet photodiode and the digital synthesizer for maintaining the modulation index signal between a pair of modulation threshold levels.